The present invention relates to a machine and process for the restrained drying of a paper web in a dryer of a paper making machine. More particularly, vacuum is supplied in the vacuum rolls located in the dry end of the dryer for restraining the web against the felt or fabric.
Paper is made by forming a web (a web is a continuously formed sheet of material) in a forming section from a slurry or "furnish" which is roughly 99% water and 1% paper pulp. As the web is formed, most of the water is separated from the web by sieving it on a conveyor belt made of wire screen, leaving a loose mat of paper fibers. The web is then pressed in a press section to remove some more of the water. The web is then guided through a dryer where it is brought into contact with a large number of rotating, steam-heated metal rollers to evaporate the remaining water from the web. How the web is formed and dried has an important bearing on the characteristics of the resulting dry paper.
The classic paper dryer consists entirely of two or more double-felted two tier dryer groups. A double tier dryer group is shown in FIG. 1. In each double-felted, double-tier dryer group there are two horizontal rows or "tiers" of dryer cylinders, one tier (dryer cylinders 11 and 12) above the other tier (dryer cylinders 14 and 15). The web is held against one dryer cylinder (14) of the lower tier, then is transferred to a dryer cylinder (11) of the upper tier, then progresses to the next dryer cylinder (15) of the lower tier, then goes to the next dryer cylinder (12) of the upper tier, and so forth.
The thermal contact between the paper and each dryer cylinder is maintained by tensioned dryer felts which apply a pressure to the paper as it wraps the dryer cylinder. Typical felt tensions range from 8 to 15 pounds per linear inch (PLI) which depending on the dryer cylinder diameter, will apply a felt pressure which is in the range of 0.22 to 0.50 PSI which is 6 to 14 inches water gauge (WG) or water column (WC) (1.5-2.5 kPa). The felt tension may be provided by a conventional felt tensioner, typically a movable roll loaded against the felt. The felt pressure not only improves the drying contact, but also applies restraint to the paper to prevent shrinkage from occurring.
This type of dryer group is called a "double-felted" group because it has two felts which alternately engage the web. The upper felt wraps around the upper part of each dryer cylinder in the upper tier, and the lower felt wraps around the lower part of each dryer cylinder in the lower tier. When the web is between the top felt and a top dryer cylinder, the bottom of the web is against the dryer cylinder. When the web is between the bottom felt and a bottom dryer cylinder, the top side of the web is against the dryer cylinder. The web is thus alternately heated on its top and bottom sides as it passes over dryer cylinders of the lower and upper tiers. In a double-felted two tier dryer portion, each dryer group is all the dryer cylinders engaging a pair of upper and lower dryer felts; a typical dryer or dryer section has several dryer groups.
As the web leaves a lower dryer cylinder to go to the next upper dryer cylinder, the felt and the lower dryer cylinder surface separate so the web can transfer to the top dryer cylinder. As the web is led away from both the lower dryer cylinder and the lower felt, it is not touching anything on either side as it follows a long path from one dryer cylinder to the other. Such a long, unsupported length of the web is called an open draw. When the web reaches the next upper dryer cylinder, the upper felt and the dryer cylinder surface come together, with the web between them, to bring the web into contact with the next upper dryer cylinder. Essentially the same procedure is followed to transfer the web from an upper dryer cylinder to a lower dryer cylinder.
In a double-felted double tier dryer, each transfer from one dryer cylinder to the next within a dryer group, or from one dryer group to the next, has required the introduction of an extended open draw, typically more than several feet (about a meter) long and at least about 16 inches (over 400 mm) long.
In a paper machine, the "machine direction" is the direction the paper is running through the length of the machine, and the "cross-machine direction" is the perpendicular direction across the width of the machine.
In open draws, the felt pressure continues to provide some restraint in the machine direction by maintaining a machine direction draw, but in the cross-machine direction, the paper is virtually unrestrained, so the sheet freely shrinks. This shrinkage is not uniform, and is greater at the edges of the sheet than in the middle, looking across the sheet. The edges shrink more because a region in the center of the sheet which shrinks is restrained by the structure of the paper on both sides, while a region on the edge which shrinks is not restrained on the outside because the paper ends at the edge. The result is uneven sheet properties across the width of the sheet, as well as edge cockles, graininess, and curl.
Cockles are defined in "The Dictionary of Paper" Fourth Edition, published 1980, as "a puckered condition of the sheet resulting from nonuniform drying and shrinking; it usually appears on paper that has had very little restraint during drying." Graininess is a large number of small variations in the surface appearance of a paper or board, resulting from any of a variety of causes, such as uneven shrinkage in drying. Curl is defined in "The Dictionary of Paper" Fourth Edition, published 1980, as "the curvature developed when one side of a paper specimen is wetted." Curl can also develop by heating the sheet, as in a photocopying machine.
The unsupported open draws also flutter, which is particularly a problem at the wet end of the dryer where the sheet is still very wet, and thus weak. Web flutter at the wet end can cause the web to break, and the machine must be cleaned out and re-threaded before it can be restored to service. The open draw thus creates runnability problems, particularly in the wet end of the dryer.
An early attempt to minimize undesirable open draws was the use of a single-felted double tier dryer group (sometimes known as a "serpentine" dryer group) in place of one of the double-felted double tier groups. In a serpentine dryer group, illustrated in FIG. 14, the dryer cylinders are again arranged in two tiers, and the web path is the same (from 100 to 104 to 101 to 105 to 102), but the web and a single felt follow the same path between the respective top and bottom cylinders.
The serpentine configuration, although reducing the problem of undesirable sheet flutter by eliminating open draws, introduces several disadvantages. First, the heat transfer from the bottom dryer cylinders is substantially reduced because the wet web is not in direct contact with the bottom cylinders. The felt is interposed between the web and the drying surfaces of the bottom cylinders. Second, the web has a tendency to sag or otherwise separate from the felt because the web travels outside the felt as it wraps around the bottom cylinder. The web can thus be influenced by the moving air and gravity, and can separate from the felt to form a bubble or flutter on the felt. Third, the initial threading of the web is not particularly easy. Further, the serpentine arrangement does not replace the open draws with positive restraint and it dries the sheet from one side only.
Others have attempted to improve the restraint applied between the top dryer cylinders of the serpentine section by inducing a vacuum along straight runs of the web by placing blow boxes next to the straight runs. But the vacuum induced in conventional serpentine blow boxes is typically only 0.1 to 0.2 inches water column (WC) (0.025-0.05 kPa), and this is clearly inadequate to provide significant shrinkage restraint. Additionally, such low level vacuum does not extend around the entire bottom dryer cylinder. With the longer sheet length between top dryer cylinders, the sheet is left unrestrained for a significant portion of the drying cycle in the conventional serpentine dryer group.
Serpentine felted two tier groups have not been continued all the way to the dry end of the dryer for an important reason--the dryer cylinders of each serpentine-felted group only directly contact one side of the web with the heated surfaces of dryer cylinders. In other words, a serpentine dryer group only directly dries one side of the paper.
Single-felted single tier construction has been developed to address some the problems of serpentine two-tier construction. In the single-felted single tier arrangement, dryer cylinders alternate with vacuum rolls and a single felt is wrapped around each dryer cylinder and vacuum roll in the group. Single-felted single tier dryer groups may be top-felted (meaning that the bottom surface of the web contacts each dryer cylinder and the felt and web run together over the top of each dryer cylinder) or bottom-felted (meaning that the top surface of the web contacts each dryer cylinder and the web and felt run together under each dryer cylinder). A portion of a "BelRun" dryer group consisting of three top-felted single tier dryer cylinders is illustrated by FIG. 15. "BelRun" is a registered trademark of Beloit Corporation.
Single-felted single tier construction is used in the wet end of the "BelRun" dryers sold by Beloit Corporation. In the "BelRun" groups, the bottom, ineffective dryer cylinders of the serpentine double-tier machine are replaced by vacuum rolls disposed below and between each pair of the dryer cylinders of a top-felted single tier of dryer cylinders. A felted run of the web passes from the preceding dryer cylinder to the vacuum roll, and then to the next dryer cylinder.
The draws between "BelRun" dryer cylinders and vacuum rolls are kept short to prevent the web from departing from the felt, and thus to eliminate or at least minimize flutter, when the web and felt pass from one dryer cylinder to the next vacuum roll. Each vacuum roll draws the web against the felt as the felt traverses the vacuum roll to restrain the web against the felt. The felt tension directly holds the felt against the dryer cylinder. The single tier dryer group thus positively keeps the web and felt together as it conveys the web through the group. Several top-felted single tier groups can be arranged in a series, with lick-down transfers between each group, so there is no open draw in the group to group transfer and there are no bottom-felted dryer cylinders in the series.
The serpentine and single tier dryer groups previously have been applied only to the wet end of the dryer. This has been done to improve runability, avoiding paper breaks and other mishaps during the manufacturing process.
The top-felted single tier dryer groups with no open draws provide improved runnability--the ability of the machine to process paper without breaks--which is particularly necessary in the wet end of the dryer. But the series of top-felted single tier dryer groups has not been continued all the way to the dry end of the dryer, because a series of like-felted (e.g. all top-felted) single-tier dryer groups connected by lickdown transfers only dries one side of the paper. This one-sided drying problem of the serpentine dryer group is not solved by providing a series of top-felted single tier dryer groups.
If one side of the paper is dried throughout the dryer, the resulting paper will have a tendency to curl as the result of heat, humidity and other factors. Paper which curls is inferior, and cannot be used for many purposes, such as photocopying or printing, where the paper must be fed through a machine after being heated or moistened. Thus, one-sided drying is discontinued in the dry end of the machine. One, more than one, or all double tier dryer groups are provided at the dry end of the dryer to reduce or substantially eliminate curl by drying both sides of the web. The shift from one-sided to two-sided drying has occurred before the web was 60% dry.
Another reason why single-felted groups have been discontinued in the dry end of the machine is that the paper is stronger there and can withstand open draws without losing its runnability.
Thus, dryer sections including multiple single-tier groups and no open draws in the wet end have been combined with conventional double felted two tier dryer groups in the dry end to complete drying. A typical dryer cylinder including single tier dryer groups has approximately 40 percent of the dryer cylinders in top-felted single tier groups, and the remaining approximately 60 percent of the dryer cylinders are two tier, double felted dryer cylinders.
One important consequence of discontinuing the single tier dryer groups before about 60% dryness (i.e. in the wet end of the dryer) is that the web was still dried substantially without restraint in the dry end (i.e. above about 60% dryness).